- Email: [email protected]
Study of plasma immersion ion implantation inside a conducting tube using an E × B field configuration
Study of plasma immersion ion implantation inside a conducting tube using an E x B field configuration Elver Juan de Dios Mitma Pillaca, Mario Ueda, Samantha de Fatima Magalh˜aes Mariano, Rogerio de Moraes Oliveira PII: DOI: Reference:
S0257-8972(14)00259-X doi: 10.1016/j.surfcoat.2014.03.042 SCT 19297
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
19 July 2013 16 December 2013 16 March 2014
Please cite this article as: Elver Juan de Dios Mitma Pillaca, Mario Ueda, Samantha de Fatima Magalh˜aes Mariano, Rogerio de Moraes Oliveira, Study of plasma immersion ion implantation inside a conducting tube using an E x B field configuration, Surface & Coatings Technology (2014), doi: 10.1016/j.surfcoat.2014.03.042
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Study of plasma immersion ion implantation inside a conducting tube using an E x B
Mario Ueda
IP
Elver Juan de Dios Mitma Pillaca
T
field configuration
SC R
Samantha de Fatima Magalhães Mariano Rogerio de Moraes Oliveira
NU
National Institute for Space Research, INPE/LAP, São Jose dos Campos, SP, Brazil Corresponding author: Elver Juan de Dios Mitma Pillaca
MA
Email: [email protected]. Tel.: +55 12 32086698 / Fax: +55 12 32086710
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Abstract
TE
Study of plasma immersion ion implantation (PIII) inside a cylindrical conductive tube
CE P
which was biased in the presence of E x B fields has been carried out. This was initiated with the electrical characterization of the system with and without the incorporation of an auxiliary electrode (AE) inside the tube. It was shown that in the presence of an AE, the
AC
breakdown of the discharge during PIII is facilitated. In the presence of the AE and the magnetic field, higher discharge currents were measured even under moderate PIII operation parameters. In order to determine the effect of implantation inside the tube, stainless steel (SS) samples were implanted with nitrogen ions with and without the presence of AE. Samples treated under these two configurations demonstrate a remarkable difference in terms of morphological, structural, tribological and mechanical properties. Higher intensity peaks of expanded austenite were detected by X-ray diffraction for samples treated in discharges with the presence of the AE. This is a consequence of higher concentration of nitrogen implanted into SS, leading to an increase of the hardness and the achievement of better corrosion resistance. 1
ACCEPTED MANUSCRIPT Keywords: Plasma immersion ion implantation in crossed E x B fields, PIII in E x B fields inner the tube, PIII with magnetic field inner the tube, PIII in E x B fields with auxiliary
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CE P
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MA
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SC R
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electrode.
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ACCEPTED MANUSCRIPT 1. Introduction The transport of corrosive and toxic fluids through the inner walls of metallic tubes is very common in chemical and petrochemical industries, causing a severe degradation of these
T
surfaces. Therefore, it is imperative to treat such surfaces in order to improve its properties,
IP
as wear and corrosion resistances, e.g. Although many surface processing technologies are
SC R
available today to modify the external surfaces of several devices, it is quite difficult to apply them to objects with concave geometries. Even for PIII, a well established method
NU
for the surface modification of 3D objects without manipulation, it is a challenge to treat pieces with geometries like concave sidewalls, since some boundaries must be overcome
MA
for this case. However, due to its great practical importance, several studies have been carried out in order to understand the behavior of the PIII inside the tubes using different
D
approaches such as analytic and numerical simulation. Recent research has shown that the
TE
impact energy of ions implanted inside the tube is significantly reduced when compared to
CE P
the one expected for convex surfaces [1]. Sheridan has attributed this behavior as dependent on the ratio between the length d of the structure of the ion-matrix sheath, and the bore radius, R, [1, 2]. He also has shown that for the
AC
condition R ≤ d, the potential drop scales as
and the ion-matrix sheath
spans the bore. However, if R > d, the sheath propagates inside the bore and the asymptotic planar solution is approached when R is greater than d. However, to raise the ion impact energy, it has been suggested recently the use of a grounded auxiliary electrode (AE) [3]. This approach is important because depending on the radius, the dose can be maximized producing a large number of ions with high impact energy bombarding the samples surfaces [3], [4].
Another way to increase the dose in PIII is the use of an external magnetic field. As suggested in previous works, the use of a non-uniform magnetic field enhances the PIII 3
ACCEPTED MANUSCRIPT due to the formation of E x B fields [5], [6]. In this configuration, the plasma density increases in the surroundings of the target, resulting in a high ion dose compared to the standard PIII [7]. Based on these results, the effect of magnetic field on PIII inside a tube
T
with and without the presence of an AE was analyzed in this work, as well as the effects of
IP
ion implantation on the inner surface of the tube. This was accomplished by positioning
SC R
stainless steel (SS) samples along the inner wall of the tube and analyzing its
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morphological, structural, and mechanical properties.
2. Experimental setup
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The experimental arrangement of PIII with magnetic field is shown in Fig 1. The experiments were performed in a cylindrical vacuum vessel with 38-cm length and 26-cm
D
diameter. A tube with 4-cm diameter and 15-cm length with a grounded AE of 0.2-cm
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radius were placed on the axial axis of the vacuum chamber. In order to produce the
CE P
required non-uniform magnetic field, the PIII vacuum chamber was equipped with a magnetic coil system to produce an axial magnetic field. It was pumped using a system of mechanical and diffusion pumps to reach a base pressure of 5x10-5 mbar and to set a
AC
nitrogen working pressure varying from 0.5 x 10-2 to 8.0 x 10-2 mbar. During the experiment the magnetic field was varied from 0 to 70 G and PIII pulse from 0 to 6 kV/ 400 Hz/35 µs. To analyze the effects of PIII in E x B fields on the tube inner wall, seven stainless steel samples were mounted on a sample holder of 15-cm length and then placed on the tube inner wall. In addition, gas inlet was directed to the inside of the tube, as shown in Fig. 1. The treatment was performed during 60 min. Due to its low cost, wide application and well-known properties, stainless steel was chosen as substrate in this experiment. So, SS disks with 0.3-cm thick and 1.5-cm diameter were used for the treatments. The analysis of samples was performed by different characterization techniques. AFM measurements were performed over an area of 5µm x 4
ACCEPTED MANUSCRIPT 5µm by a NanoScope V microscope, operated in the tapping mode, in order to analyze surface topography and roughness (Ra). The topography analysis was performed by scanning electron microscopy (SEM) using an electron beam of 20 keV energy, scanning
T
area of 1.0 mm x 1.0 mm with magnification of 1000x. The structural changes in the
IP
surface layer were investigated by X-ray diffraction (XRD) in a Bragg-Brentano geometry
SC R
with CuKα radiation. Tribological measurements were accomplished in a CSMinstruments Pin-on-disk Tribometer. The Tribometer was operated at load of 1.0 N in air
NU
atmosphere with relative humidity of about 60 %, using an alumina ball of 3.0 mm diameter and fixed linear velocity of 5 cm/s. Surface hardness was evaluated by
MA
microindentation using loads of 10 gf. The variation of the temperature of the tube during PIII was measured by an optical pyrometer (Micron M90). Finally, analysis of corrosion
D
was performed by using potentiodynamic polarization test. Profile of polarization curves
TE
was obtained by using an IVIUMSTAR electrochemical system. Corrosion studies were
3. Results
CE P
performed in 3.5 % NaCl solution.
AC
All the experiments were performed with the presence of the magnetic field due to enhanced plasma stability for such cases, which is more evident for lower pressure values.
3.1. Electric characterization of PIII in E x B fields inside the tube Fig. 2 shows the results of the breakdown voltage as function of the nitrogen gas pressure (from 0.5 x 10-2 mbar to 8.0 x 10-2 mbar) when applying 45 G of magnetic field. Our experimental results show that the gas breakdown is more difficult to be obtained in low pressures when the magnetic field is absent. The presence of the AE helped to reduce the minimum breakdown voltage, from 5.5 kV to about 2.5 kV for lower pressures. As the pressure is increased there is a tendency to reach the breakdown at lower voltages for all 5
ACCEPTED MANUSCRIPT the cases. However, it is important to impose an upper limit on the working pressure in order to avoid a regime of collisional sheath, which would reduce the ion impact energy
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during PIII [8].
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We analyzed the effects of the magnetic field on the current of the PIII operating at 7.0 x
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10-3 mbar (adequate for PIII) and voltage of 3 kV. The behavior of the total current as a function of magnetic field intensity is shown in Fig. 3. The profiles are completely
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different in this case compared to that one obtained for standard PIII [6]. When the magnetic field is varied in the presence of an AE, the current intensity increases quickly, as
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can be seen in Fig. 3a. In addition, we noted that it begins each time earlier before the pulse end. Another interesting characteristic is that the plateau region of the profile shows
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a same value. These results are not observed in the absence of an auxiliary electrode. In
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this case, the currents start from the same point, increase only its amplitude, as is shown in
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Fig. 3b.
3.2. Effects of PIII in E x B configuration inside the tube
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The effects of PIII in E x B inside the tube were analyzed after nitrogen ion implantation with and without the presence of the AE. The treatment was carried out at pressure of 2.0 x 10-2 mbar and voltage of 4 kV. The magnetic field was adjusted between 50 G and 60 G to keep nearly the same current value for both cases. The characterizations were performed for samples placed in the center of the sample-holder.
3.2.1. Morphology of surface The effect of nitrogen implantation on substrate surface was investigated using SEM and AFM images. These images are shown in Fig. 4 and the values of surface roughness obtained by AFM are reported in Table I. A substantial presence of holes (manufacture 6
ACCEPTED MANUSCRIPT defect) are exhibited by SEM in Fig. 4a for untreated samples. It is possible to see a substantial change in surface morphology after the treatment performed with the presence of an AE. Here, the grain contours were revealed, exhibiting its classical polycrystalline
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structure (Fig. 4c). In this case, it can be attributed to an intense ion bombardment taking
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place inner the tube wall. High current of 4 A measured during the treatment is the cause of
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an intense surface sputtering. This can be related to the bombardment of ions with higher energies in the presence of the AE, such as it is predicted by theory [3]. The same situation
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was also reported in a previous work [7] using E x B fields in a parallelepiped target. Changes were also observed for the case without AE for which a smoother surface
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finishing was found (see Fig. 4b). We believe that in this situation the treatment was less
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intense than to the case with AE. Probably, ions hit the wall with lower energies [3].
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The surface morphologies of untreated SS sample and of samples treated without and with
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the AE are shown in Figures 4d, 4e and 4f, respectively. An average roughness of Ra 1.3 nm was measured for the untreated sample, a similar value measured for the sample treated without the AE, as shown in Table I. However, when the discharge is switched-on with the
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AE, Ra raised to about 10.7 +/- 1.5 nm. This increase may be due to the higher sputtering rate caused by higher ions flux hitting the substrate during PIII.
Tribology results shown in Table I reveal a significant reduction of friction coefficient (µ) from µ = 0.8 to µ = 0.6 for samples treated in discharge with the presence of the AE. However, when the AE was not used, the value of µ has not experienced much change, remaining similar to the untreated one.
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ACCEPTED MANUSCRIPT 3.2.2 Structural and mechanical properties Fig. 5 depicts XRD diffractograms for reference and treated samples with and without the presence of an AE. Our results in Fig. 5 show the presence of new peaks next to the 111
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and 200 characteristic ones. This result indicates the formation of a metastable expanded
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austenite phase, rich in nitrogen, called N phase [9], [10], [11]. Here, formation of N
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phase is a consequence of the N thermal diffusion caused by the high incidence of ions during the treatment. For the treated sample without AE a low intensity can be noted.
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However, it shows a high intensity with the presence of AE. In this case, a temperature of 360 oC was achieved at the end of the treatment, above the 320 °C measured without AE. It
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increase of intensity of the N peak.
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was also observed that localized nitrogen gas injection inside the tube results in further
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Results of microhardness tests are shown in Table I. Improvement in surface hardness is
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observed for each implanted sample. Here, a relative increase of hardness resulted when the samples were treated in the presence of an AE, especially for localized gas injection. For this case, the hardness increased twofold. We believe that these results are a
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consequence of the presence of the austenitic expanded phase, rich in nitrogen.
3.2.3 Corrosion Fig. 6 shows the potentiodynamic polarization profiles for the standard and treated samples by PIII, in E x B, with and without AE. The corrosion tests for the treated samples show the polarization curve shifted to the left compared to reference sample. In this region, the current density is about five times less than for the untreated sample. This result indicates that the corrosion resistance has been improved due to modification of other surface properties.
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ACCEPTED MANUSCRIPT 4. Discussion Our study of breakdown voltage in E x B showed that to produce the plasma inside the tube in this case only low voltages are required. This is important because it enables one to
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extend the range of PIII operation and supply higher energy to ions. At low gas pressure,
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the effect of electrons drifting with cyclotron frequency becomes dominant compared to
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the frequency of gas collision, . However, it decreases at high pressures. In this last
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condition, the magnetic field does not play a significant role.
In the > scenario, the magnetic field was varied in the presence of an AE. Here, the
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behavior of the current changing quickly for high B is more evident towards the region near the end of the displacement current. This delay time for the high-current may be
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understood analyzing the dynamic of the electrons in presence of both, the radial electric
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field improved by the presence of the AE and the axial magnetic field. Thus, when B is
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increased, the number of collisions between electron and neutral species increase since more electrons will be drifting in E x B direction. This leads to an additional increase of the gas ionization (ions + electrons) with enhanced confinement of the plasma provided by
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the magnetic field [5], [6]. As a result there is an increase of the plasma density enabling an increase of the extracted current, as observed in Fig. 3a. Another cause of the current increase may be attributed to the higher emission of secondary electrons due to more frequent collisions of higher energetic ions with the inner wall.
Continuing the discussion, one real problem of the PIII in tubes is the edge effect. Although an AE is used in our experiment, the strength of the electric field remains high enough to affect the dynamics of the electrons. It makes possible that energetic electrons escape from the magnetic mirror. In fact, it can happen because electrons moving in axial direction are leaving freely the tube outward the magnetic trap. We verified that the Bmax 9
ACCEPTED MANUSCRIPT is located beside the end of the tube where the Bmin/Bmax ratio is only about 0.1. Although the ratio of the magnetic field in this magnetic bottle configuration is low, it is enough to enhance the PIII process inside the tube. Some electrons escaping from inside
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the tube follow the magnetic field line toward the outside of tube to contribute with the
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ionization of the gas. This can explain why there is plasma formation out of the tube. Our
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measurements indicate that most ions that contribute to the total current are coming from inside the tube. It is evident from the plasma light emission which is more intense from
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inside the tube than from the outside.
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In our experiment, the total current measured during the treatment shows a high value (about 4 A) for both cases, with and without AE. According to the reference [1], we can
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estimate the plasma density from the ion-matrix overload using our experimental
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parameters: 0.02 meter of tube radius and voltage applied of 4 kV. With these values a
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high density exceeding the 2 x 1015 m-3 is expected. With this elevated number of ions bombarding inside the tube wall, with AE, an evident change on the surface morphology of the SS test samples has been caused (seen Fig. 4c and Fig. 4f). As a consequence of it, the
AC
grain boundaries were revealed, causing an eightfold increase of its roughness. However, with the modified surface, the wear rate decreased significantly. Another consequence of the high energy ions hitting the tube wall, is the strong rise of the temperature. That caused significant ion diffusion in the sample, modifying its internal structure due to the formation of a new phase, N (see Fig. 5). This new phase improved significantly the hardness of the sample. The modifications of the surface morphology of treated samples, such as the exposure of the grain boundary, did not let the surface susceptible to corrosion. On the contrary, it seems that the presence of the expanded austenite in this case favored the improvement of the resistance to corrosion. 10
ACCEPTED MANUSCRIPT 5. Conclusions Electric characterization of PIII in E x B fields inside the tube was investigated with and without a grounded AE. Our results show that E x B fields facilitates the plasma formation
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in conditions of lower voltages and lower pressures. This result is more evident when an
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auxiliary electrode is used. In this case, a high current of about 4 A was obtained by using
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a magnetic field of 60 G. To study the effectiveness of ion implantation inside the tube with and without the presence of an AE, SS samples were used for testing.
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Characterization results for samples treated by PIII with the presence of the AE revealed significant changes of morphological, mechanical and structural properties. This can be
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attributed to the presence of a rich nitrogen layer into SS caused by the thermal diffusion of implanted ions. It was also observed that N peaks were intensified with a localized gas
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injection, probably due to an increase of the ions dose implanted into SS. In conclusion,
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PIII inside a conducting tube using E x B field configuration with auxiliary electrode has
6. References
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been demonstrated to be well suitable for the treatment inside cylindrical tubes.
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[1] T. E. Sheridan, J. Appl. Phys, 80, (1996). [2] T. E. Sheridan, Phys. Plasmas, 1 (1994). [3] X. Zeng, B. Tang, P. K. Chu, Appl. Phys. Lett, 69 (1996). [4] D. T. Kwok, X. Zeng, Q. Chen, P. K. Chu, T. E. Sheridan, IEEE Transactions on Plasma Science, 27 (1999). [5] K. G. Kostov, M. A. Algatti, E. J. D. M. Pillaca, M. E. Kayama, R. P. Mota and R. Y. Honda, Eur. Phys. J. D, 54 (2009). [6] E. J. D. M. Pillaca, M. Ueda, K. G. Kostov, IEEE Transactions on Plasma Science, 39 (2011). [7] E. J. D. M. Pillaca, M. Ueda, K. G. Kostov, H. Reuther, Applied Surface Science 258 (2012). [8] T. E. Sheridan, M. J. Goeckner, J. Appl. Phys, 77 (1995).
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ACCEPTED MANUSCRIPT [9] T. Christiansen and M. A. J. Somers, Metallurgical and Materials Transactions A, 37A, (2006). [10] C. B. Mello, M. Ueda, C. M. Lepienski, H. Reuther, Surface and Coatings Technology, 256 (2009).
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[11] Oliveira, R. M., Ueda, M., Silva, L. L. G., Reuther, H., C. M. Lepienski. Brazilian
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Journal of Physics, v. 39, p. 554-558, (2009).
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ACCEPTED MANUSCRIPT Captions of figures
Fig. 1. Schematic diagram of the vacuum chamber with magnetic coils covering the tube, which is crossed by an auxiliary electrode.
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Fig. 2. Breakdown voltage as a function of the gas pressure with and without magnetic field. Here, tubes with and without auxiliary electrode were used for the case without magnetic field.
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Fig. 3. Time-dependent current profiles as function of the magnetic field at a gas pressure of 7.0x10-3 mbar and at applied voltage of 3 kV. (a) With auxiliary electrode (b) Without auxiliary electrode.
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Fig. 4. Surface morphology of 304SS samples. SEM and AFM: (a) and (d) untreated, (b) and (e) treated without auxiliary electrode, (c) and (f) Treated with auxiliary electrode.
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Fig. 5. X-ray diffraction patterns for the stainless steel samples for the reference sample and treated samples after 60 min.
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Fig. 6. Typical polarization corrosion curves of stainless steel for the untreated and treated samples.
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ACCEPTED MANUSCRIPT Table I. Results of tests obtained for reference and treated samples with and without AE* and localized gas injection. Friction coefficient (µ)
1.3 +/- 0.2
0.8
Without AE
2.9 +/- 0.1
0.8
With AE
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Reference
10.7 +/- 1.5
With AE/gas injection.
213.5 +/- 1.5
231.6 +/- 12.1
0.6
276 +/- 7.1
-
550 +/- 80
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-
Hardness (Hv)
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Roughness (nm)
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Condition
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*AE: auxiliary electrode
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ACCEPTED MANUSCRIPT Highlights
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PIII in ExB fields with auxiliary electrode (AE) is more stable than without AE High ion dose in the tube inner wall by PIII has improved its surface properties Corrosion resistance inside the tube was improved after PIII in ExB fields Plasma formation inside the tube is facilitated at lower voltage and higher pressures
22
S0257-8972(14)00259-X doi: 10.1016/j.surfcoat.2014.03.042 SCT 19297
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
19 July 2013 16 December 2013 16 March 2014
Please cite this article as: Elver Juan de Dios Mitma Pillaca, Mario Ueda, Samantha de Fatima Magalh˜aes Mariano, Rogerio de Moraes Oliveira, Study of plasma immersion ion implantation inside a conducting tube using an E x B field configuration, Surface & Coatings Technology (2014), doi: 10.1016/j.surfcoat.2014.03.042
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Study of plasma immersion ion implantation inside a conducting tube using an E x B
Mario Ueda
IP
Elver Juan de Dios Mitma Pillaca
T
field configuration
SC R
Samantha de Fatima Magalhães Mariano Rogerio de Moraes Oliveira
NU
National Institute for Space Research, INPE/LAP, São Jose dos Campos, SP, Brazil Corresponding author: Elver Juan de Dios Mitma Pillaca
MA
Email: [email protected]. Tel.: +55 12 32086698 / Fax: +55 12 32086710
D
Abstract
TE
Study of plasma immersion ion implantation (PIII) inside a cylindrical conductive tube
CE P
which was biased in the presence of E x B fields has been carried out. This was initiated with the electrical characterization of the system with and without the incorporation of an auxiliary electrode (AE) inside the tube. It was shown that in the presence of an AE, the
AC
breakdown of the discharge during PIII is facilitated. In the presence of the AE and the magnetic field, higher discharge currents were measured even under moderate PIII operation parameters. In order to determine the effect of implantation inside the tube, stainless steel (SS) samples were implanted with nitrogen ions with and without the presence of AE. Samples treated under these two configurations demonstrate a remarkable difference in terms of morphological, structural, tribological and mechanical properties. Higher intensity peaks of expanded austenite were detected by X-ray diffraction for samples treated in discharges with the presence of the AE. This is a consequence of higher concentration of nitrogen implanted into SS, leading to an increase of the hardness and the achievement of better corrosion resistance. 1
ACCEPTED MANUSCRIPT Keywords: Plasma immersion ion implantation in crossed E x B fields, PIII in E x B fields inner the tube, PIII with magnetic field inner the tube, PIII in E x B fields with auxiliary
AC
CE P
TE
D
MA
NU
SC R
IP
T
electrode.
2
ACCEPTED MANUSCRIPT 1. Introduction The transport of corrosive and toxic fluids through the inner walls of metallic tubes is very common in chemical and petrochemical industries, causing a severe degradation of these
T
surfaces. Therefore, it is imperative to treat such surfaces in order to improve its properties,
IP
as wear and corrosion resistances, e.g. Although many surface processing technologies are
SC R
available today to modify the external surfaces of several devices, it is quite difficult to apply them to objects with concave geometries. Even for PIII, a well established method
NU
for the surface modification of 3D objects without manipulation, it is a challenge to treat pieces with geometries like concave sidewalls, since some boundaries must be overcome
MA
for this case. However, due to its great practical importance, several studies have been carried out in order to understand the behavior of the PIII inside the tubes using different
D
approaches such as analytic and numerical simulation. Recent research has shown that the
TE
impact energy of ions implanted inside the tube is significantly reduced when compared to
CE P
the one expected for convex surfaces [1]. Sheridan has attributed this behavior as dependent on the ratio between the length d of the structure of the ion-matrix sheath, and the bore radius, R, [1, 2]. He also has shown that for the
AC
condition R ≤ d, the potential drop scales as
and the ion-matrix sheath
spans the bore. However, if R > d, the sheath propagates inside the bore and the asymptotic planar solution is approached when R is greater than d. However, to raise the ion impact energy, it has been suggested recently the use of a grounded auxiliary electrode (AE) [3]. This approach is important because depending on the radius, the dose can be maximized producing a large number of ions with high impact energy bombarding the samples surfaces [3], [4].
Another way to increase the dose in PIII is the use of an external magnetic field. As suggested in previous works, the use of a non-uniform magnetic field enhances the PIII 3
ACCEPTED MANUSCRIPT due to the formation of E x B fields [5], [6]. In this configuration, the plasma density increases in the surroundings of the target, resulting in a high ion dose compared to the standard PIII [7]. Based on these results, the effect of magnetic field on PIII inside a tube
T
with and without the presence of an AE was analyzed in this work, as well as the effects of
IP
ion implantation on the inner surface of the tube. This was accomplished by positioning
SC R
stainless steel (SS) samples along the inner wall of the tube and analyzing its
NU
morphological, structural, and mechanical properties.
2. Experimental setup
MA
The experimental arrangement of PIII with magnetic field is shown in Fig 1. The experiments were performed in a cylindrical vacuum vessel with 38-cm length and 26-cm
D
diameter. A tube with 4-cm diameter and 15-cm length with a grounded AE of 0.2-cm
TE
radius were placed on the axial axis of the vacuum chamber. In order to produce the
CE P
required non-uniform magnetic field, the PIII vacuum chamber was equipped with a magnetic coil system to produce an axial magnetic field. It was pumped using a system of mechanical and diffusion pumps to reach a base pressure of 5x10-5 mbar and to set a
AC
nitrogen working pressure varying from 0.5 x 10-2 to 8.0 x 10-2 mbar. During the experiment the magnetic field was varied from 0 to 70 G and PIII pulse from 0 to 6 kV/ 400 Hz/35 µs. To analyze the effects of PIII in E x B fields on the tube inner wall, seven stainless steel samples were mounted on a sample holder of 15-cm length and then placed on the tube inner wall. In addition, gas inlet was directed to the inside of the tube, as shown in Fig. 1. The treatment was performed during 60 min. Due to its low cost, wide application and well-known properties, stainless steel was chosen as substrate in this experiment. So, SS disks with 0.3-cm thick and 1.5-cm diameter were used for the treatments. The analysis of samples was performed by different characterization techniques. AFM measurements were performed over an area of 5µm x 4
ACCEPTED MANUSCRIPT 5µm by a NanoScope V microscope, operated in the tapping mode, in order to analyze surface topography and roughness (Ra). The topography analysis was performed by scanning electron microscopy (SEM) using an electron beam of 20 keV energy, scanning
T
area of 1.0 mm x 1.0 mm with magnification of 1000x. The structural changes in the
IP
surface layer were investigated by X-ray diffraction (XRD) in a Bragg-Brentano geometry
SC R
with CuKα radiation. Tribological measurements were accomplished in a CSMinstruments Pin-on-disk Tribometer. The Tribometer was operated at load of 1.0 N in air
NU
atmosphere with relative humidity of about 60 %, using an alumina ball of 3.0 mm diameter and fixed linear velocity of 5 cm/s. Surface hardness was evaluated by
MA
microindentation using loads of 10 gf. The variation of the temperature of the tube during PIII was measured by an optical pyrometer (Micron M90). Finally, analysis of corrosion
D
was performed by using potentiodynamic polarization test. Profile of polarization curves
TE
was obtained by using an IVIUMSTAR electrochemical system. Corrosion studies were
3. Results
CE P
performed in 3.5 % NaCl solution.
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All the experiments were performed with the presence of the magnetic field due to enhanced plasma stability for such cases, which is more evident for lower pressure values.
3.1. Electric characterization of PIII in E x B fields inside the tube Fig. 2 shows the results of the breakdown voltage as function of the nitrogen gas pressure (from 0.5 x 10-2 mbar to 8.0 x 10-2 mbar) when applying 45 G of magnetic field. Our experimental results show that the gas breakdown is more difficult to be obtained in low pressures when the magnetic field is absent. The presence of the AE helped to reduce the minimum breakdown voltage, from 5.5 kV to about 2.5 kV for lower pressures. As the pressure is increased there is a tendency to reach the breakdown at lower voltages for all 5
ACCEPTED MANUSCRIPT the cases. However, it is important to impose an upper limit on the working pressure in order to avoid a regime of collisional sheath, which would reduce the ion impact energy
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during PIII [8].
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We analyzed the effects of the magnetic field on the current of the PIII operating at 7.0 x
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10-3 mbar (adequate for PIII) and voltage of 3 kV. The behavior of the total current as a function of magnetic field intensity is shown in Fig. 3. The profiles are completely
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different in this case compared to that one obtained for standard PIII [6]. When the magnetic field is varied in the presence of an AE, the current intensity increases quickly, as
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can be seen in Fig. 3a. In addition, we noted that it begins each time earlier before the pulse end. Another interesting characteristic is that the plateau region of the profile shows
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a same value. These results are not observed in the absence of an auxiliary electrode. In
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this case, the currents start from the same point, increase only its amplitude, as is shown in
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3.2. Effects of PIII in E x B configuration inside the tube
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The effects of PIII in E x B inside the tube were analyzed after nitrogen ion implantation with and without the presence of the AE. The treatment was carried out at pressure of 2.0 x 10-2 mbar and voltage of 4 kV. The magnetic field was adjusted between 50 G and 60 G to keep nearly the same current value for both cases. The characterizations were performed for samples placed in the center of the sample-holder.
3.2.1. Morphology of surface The effect of nitrogen implantation on substrate surface was investigated using SEM and AFM images. These images are shown in Fig. 4 and the values of surface roughness obtained by AFM are reported in Table I. A substantial presence of holes (manufacture 6
ACCEPTED MANUSCRIPT defect) are exhibited by SEM in Fig. 4a for untreated samples. It is possible to see a substantial change in surface morphology after the treatment performed with the presence of an AE. Here, the grain contours were revealed, exhibiting its classical polycrystalline
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structure (Fig. 4c). In this case, it can be attributed to an intense ion bombardment taking
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place inner the tube wall. High current of 4 A measured during the treatment is the cause of
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an intense surface sputtering. This can be related to the bombardment of ions with higher energies in the presence of the AE, such as it is predicted by theory [3]. The same situation
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was also reported in a previous work [7] using E x B fields in a parallelepiped target. Changes were also observed for the case without AE for which a smoother surface
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finishing was found (see Fig. 4b). We believe that in this situation the treatment was less
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The surface morphologies of untreated SS sample and of samples treated without and with
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the AE are shown in Figures 4d, 4e and 4f, respectively. An average roughness of Ra 1.3 nm was measured for the untreated sample, a similar value measured for the sample treated without the AE, as shown in Table I. However, when the discharge is switched-on with the
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AE, Ra raised to about 10.7 +/- 1.5 nm. This increase may be due to the higher sputtering rate caused by higher ions flux hitting the substrate during PIII.
Tribology results shown in Table I reveal a significant reduction of friction coefficient (µ) from µ = 0.8 to µ = 0.6 for samples treated in discharge with the presence of the AE. However, when the AE was not used, the value of µ has not experienced much change, remaining similar to the untreated one.
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austenite phase, rich in nitrogen, called N phase [9], [10], [11]. Here, formation of N
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phase is a consequence of the N thermal diffusion caused by the high incidence of ions during the treatment. For the treated sample without AE a low intensity can be noted.
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However, it shows a high intensity with the presence of AE. In this case, a temperature of 360 oC was achieved at the end of the treatment, above the 320 °C measured without AE. It
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increase of intensity of the N peak.
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was also observed that localized nitrogen gas injection inside the tube results in further
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Results of microhardness tests are shown in Table I. Improvement in surface hardness is
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observed for each implanted sample. Here, a relative increase of hardness resulted when the samples were treated in the presence of an AE, especially for localized gas injection. For this case, the hardness increased twofold. We believe that these results are a
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consequence of the presence of the austenitic expanded phase, rich in nitrogen.
3.2.3 Corrosion Fig. 6 shows the potentiodynamic polarization profiles for the standard and treated samples by PIII, in E x B, with and without AE. The corrosion tests for the treated samples show the polarization curve shifted to the left compared to reference sample. In this region, the current density is about five times less than for the untreated sample. This result indicates that the corrosion resistance has been improved due to modification of other surface properties.
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ACCEPTED MANUSCRIPT 4. Discussion Our study of breakdown voltage in E x B showed that to produce the plasma inside the tube in this case only low voltages are required. This is important because it enables one to
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extend the range of PIII operation and supply higher energy to ions. At low gas pressure,
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the effect of electrons drifting with cyclotron frequency becomes dominant compared to
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the frequency of gas collision, . However, it decreases at high pressures. In this last
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condition, the magnetic field does not play a significant role.
In the > scenario, the magnetic field was varied in the presence of an AE. Here, the
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behavior of the current changing quickly for high B is more evident towards the region near the end of the displacement current. This delay time for the high-current may be
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understood analyzing the dynamic of the electrons in presence of both, the radial electric
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field improved by the presence of the AE and the axial magnetic field. Thus, when B is
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increased, the number of collisions between electron and neutral species increase since more electrons will be drifting in E x B direction. This leads to an additional increase of the gas ionization (ions + electrons) with enhanced confinement of the plasma provided by
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the magnetic field [5], [6]. As a result there is an increase of the plasma density enabling an increase of the extracted current, as observed in Fig. 3a. Another cause of the current increase may be attributed to the higher emission of secondary electrons due to more frequent collisions of higher energetic ions with the inner wall.
Continuing the discussion, one real problem of the PIII in tubes is the edge effect. Although an AE is used in our experiment, the strength of the electric field remains high enough to affect the dynamics of the electrons. It makes possible that energetic electrons escape from the magnetic mirror. In fact, it can happen because electrons moving in axial direction are leaving freely the tube outward the magnetic trap. We verified that the Bmax 9
ACCEPTED MANUSCRIPT is located beside the end of the tube where the Bmin/Bmax ratio is only about 0.1. Although the ratio of the magnetic field in this magnetic bottle configuration is low, it is enough to enhance the PIII process inside the tube. Some electrons escaping from inside
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the tube follow the magnetic field line toward the outside of tube to contribute with the
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ionization of the gas. This can explain why there is plasma formation out of the tube. Our
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measurements indicate that most ions that contribute to the total current are coming from inside the tube. It is evident from the plasma light emission which is more intense from
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inside the tube than from the outside.
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In our experiment, the total current measured during the treatment shows a high value (about 4 A) for both cases, with and without AE. According to the reference [1], we can
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estimate the plasma density from the ion-matrix overload using our experimental
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parameters: 0.02 meter of tube radius and voltage applied of 4 kV. With these values a
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high density exceeding the 2 x 1015 m-3 is expected. With this elevated number of ions bombarding inside the tube wall, with AE, an evident change on the surface morphology of the SS test samples has been caused (seen Fig. 4c and Fig. 4f). As a consequence of it, the
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grain boundaries were revealed, causing an eightfold increase of its roughness. However, with the modified surface, the wear rate decreased significantly. Another consequence of the high energy ions hitting the tube wall, is the strong rise of the temperature. That caused significant ion diffusion in the sample, modifying its internal structure due to the formation of a new phase, N (see Fig. 5). This new phase improved significantly the hardness of the sample. The modifications of the surface morphology of treated samples, such as the exposure of the grain boundary, did not let the surface susceptible to corrosion. On the contrary, it seems that the presence of the expanded austenite in this case favored the improvement of the resistance to corrosion. 10
ACCEPTED MANUSCRIPT 5. Conclusions Electric characterization of PIII in E x B fields inside the tube was investigated with and without a grounded AE. Our results show that E x B fields facilitates the plasma formation
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in conditions of lower voltages and lower pressures. This result is more evident when an
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auxiliary electrode is used. In this case, a high current of about 4 A was obtained by using
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a magnetic field of 60 G. To study the effectiveness of ion implantation inside the tube with and without the presence of an AE, SS samples were used for testing.
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Characterization results for samples treated by PIII with the presence of the AE revealed significant changes of morphological, mechanical and structural properties. This can be
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attributed to the presence of a rich nitrogen layer into SS caused by the thermal diffusion of implanted ions. It was also observed that N peaks were intensified with a localized gas
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injection, probably due to an increase of the ions dose implanted into SS. In conclusion,
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PIII inside a conducting tube using E x B field configuration with auxiliary electrode has
6. References
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been demonstrated to be well suitable for the treatment inside cylindrical tubes.
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[1] T. E. Sheridan, J. Appl. Phys, 80, (1996). [2] T. E. Sheridan, Phys. Plasmas, 1 (1994). [3] X. Zeng, B. Tang, P. K. Chu, Appl. Phys. Lett, 69 (1996). [4] D. T. Kwok, X. Zeng, Q. Chen, P. K. Chu, T. E. Sheridan, IEEE Transactions on Plasma Science, 27 (1999). [5] K. G. Kostov, M. A. Algatti, E. J. D. M. Pillaca, M. E. Kayama, R. P. Mota and R. Y. Honda, Eur. Phys. J. D, 54 (2009). [6] E. J. D. M. Pillaca, M. Ueda, K. G. Kostov, IEEE Transactions on Plasma Science, 39 (2011). [7] E. J. D. M. Pillaca, M. Ueda, K. G. Kostov, H. Reuther, Applied Surface Science 258 (2012). [8] T. E. Sheridan, M. J. Goeckner, J. Appl. Phys, 77 (1995).
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ACCEPTED MANUSCRIPT [9] T. Christiansen and M. A. J. Somers, Metallurgical and Materials Transactions A, 37A, (2006). [10] C. B. Mello, M. Ueda, C. M. Lepienski, H. Reuther, Surface and Coatings Technology, 256 (2009).
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[11] Oliveira, R. M., Ueda, M., Silva, L. L. G., Reuther, H., C. M. Lepienski. Brazilian
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Journal of Physics, v. 39, p. 554-558, (2009).
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ACCEPTED MANUSCRIPT Captions of figures
Fig. 1. Schematic diagram of the vacuum chamber with magnetic coils covering the tube, which is crossed by an auxiliary electrode.
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Fig. 2. Breakdown voltage as a function of the gas pressure with and without magnetic field. Here, tubes with and without auxiliary electrode were used for the case without magnetic field.
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Fig. 3. Time-dependent current profiles as function of the magnetic field at a gas pressure of 7.0x10-3 mbar and at applied voltage of 3 kV. (a) With auxiliary electrode (b) Without auxiliary electrode.
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Fig. 4. Surface morphology of 304SS samples. SEM and AFM: (a) and (d) untreated, (b) and (e) treated without auxiliary electrode, (c) and (f) Treated with auxiliary electrode.
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Fig. 5. X-ray diffraction patterns for the stainless steel samples for the reference sample and treated samples after 60 min.
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Fig. 6. Typical polarization corrosion curves of stainless steel for the untreated and treated samples.
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ACCEPTED MANUSCRIPT Table I. Results of tests obtained for reference and treated samples with and without AE* and localized gas injection. Friction coefficient (µ)
1.3 +/- 0.2
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Without AE
2.9 +/- 0.1
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With AE
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10.7 +/- 1.5
With AE/gas injection.
213.5 +/- 1.5
231.6 +/- 12.1
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276 +/- 7.1
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550 +/- 80
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Hardness (Hv)
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Roughness (nm)
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*AE: auxiliary electrode
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ACCEPTED MANUSCRIPT Highlights
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PIII in ExB fields with auxiliary electrode (AE) is more stable than without AE High ion dose in the tube inner wall by PIII has improved its surface properties Corrosion resistance inside the tube was improved after PIII in ExB fields Plasma formation inside the tube is facilitated at lower voltage and higher pressures
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